Recent progresses in the field of Light Emitting Diodes (LEDs) have opened the field to new families of applications.
Improvements of internal quantum efficiency and improvement of the extraction efficiency strategies have led to LED chips that deliver high flux. By using high performance packages, LED-based devices at higher driving currents have been developed. Consequently, performance and reliability of LED-based devices have reached new standards, opening ways for high brightness, color-sensitive applications. With this level of performance, LEDs have become a viable alternative to Cold Cathode Fluorescent Lamp (CCFL) for LCD backlighting.
Compared to CCFL, LEDs offer a new level of flexibility to LCD designers. New generations of LEDs promise to reshape and eventually reduce power consumption. They also offer the possibility of an independent control of various combinations to produce the desired lighting effect.
For example, the response time of LEDs is much faster than the response time of CCFL and obviously much faster than the typical response time of the human eye. It is therefore possible to modulate sequentially the different LED chips to obtain the desired optical properties in terms of light level or color rendering. These factors contribute to the adoption of LEDs for LCD backlighting in place of CCFL. However some issues remain.
The lenses described in prior art for single chip devices are suited for operation with a single on-axis light-emitting chip (U.S. Pat. Nos. 6,598,998/6,607,286/6,679,621). The dimensions of the lenses are typically 7 mm in diameter and 4 mm in height. Any variation of the position of the chip leads to a failure of the Total Internal Reflection (TIR) principle and undesired redirection of light outside of the desired angle for both TIR and reflective-coating designs.
Moreover single chip devices present several limitations for backlighting applications. For example each device emits a single color. When multiple single chip LED devices are used, the color mixing elements of the LCD backlighting system must take up the heavy burden of controlling the mixing of the different color light generated by the multiple single chip LED devices to deliver light of the desired color. Moreover, a LCD with multiple single-chip devices requires several layers of lightguide, light diffusers, and light shapers. Each of these elements reduces the overall light transmission of the system.
Multichip and multicolor LED devices perform a localized color mixing within the devices by controlling the light emission of the LED devices. This feature reduces the number of light diffusers and light shapers inside the LCD system. The transmittance of the system is improved as well as the overall brightness on the projection screen.
For multichip designs, the chips are not centered on the optical axis of the lens. Performances as low as 10-20% of light redirected laterally would be obtained with a lens for a single chip system. The major reason for the drop in performance is that light is emitted from a much wider area that can be at least 4 times larger than that of a single chip.
One solution can be to scale-up the dimensions of the optical lens. For example, for a 4-chip design the dimension of the lens can be multiplied by a factor 4 to keep the same level of performance. It means the dimensions of the lens would be typically 28 mm in diameter and 16 mm in height. This would be a barrier for any LCD application. The size, the weight and the cost of the lens would be far beyond the requirements of LCD TV and projectors.
It is therefore desirable to provide a new optical system that redirects and couples an optimal amount of light emitted by multiple light emitting diode chips into a lateral lightguide and therefore provide uniform illumination on the LCD screen.